Since electronic control technology has come to be used widely for controlling vehicles and vehicle engines, many methods have been proposed for dealing with knocking. In addition to improving the shape of the combustion chamber, improving gas flow and prevention of knocking by increasing the octane value of the fuel, knocking control techniques which aim to provide optimum fuel cost performance or dynamic performance for each running condition irrespective of differences in the nature of the fuel and time-dependent variations of the octane value required by the engine, are coming to be applied to mass-produced vehicles by advancing the ignition period to a limit at which knocking can hardly be heard by the human ear. Some of these techniques are now being applied to each cylinder of six cylinder engines.
The detection and quantization of knocking are important factors in onboard knocking control techniques using electronics. The knocking control devices installed in an automobile might for example use cylinder internal pressure, engine vibration, combustion light, knocking noise and ion current in the cylinder as possible knocking detection parameters.
One such detector is disclosed in Vol. 40, No. 11 of Jidosha Gijutsu ("Automobile Technology") published in Japan in 1986. This detector comprises an cylinder internal pressure sensor mounted on the spark plug for detecting the pressure in the combustion chamber, extracting high frequency components in the region of the knocking frequency from the waveform pattern of the cylinder internal pressure found from the output of this pressure sensor, converting the vibration intensity to a number, and using statistical processing to determine a knocking level.
However, in the aforesaid knocking detector, the detection results are easily influenced by the position and specification of the cylinder internal pressure sensor, or according to the cylinder in which the knocking detection is performed. To eliminate this influence the knocking detection logic had to be optimized for each type of engine which increased the number of operations and cost involved, and knocking detection precision was impaired if the logic was not properly optimized.
Further, although the cylinder internal pressure sensor mounted on the spark plug has the advantage that no special modification of the engine is required, it also picks up mechanical vibrations of the engine in addition to the combustion pressure. If high frequency signal components of the sensor output waveform are used, it is difficult to separate frequencies due to noise, and it was therefore difficult to increase the detection precision of knocking components. In particular, as high frequency vibrations increase when the engine is running at high speed, the detection of knocking components if rendered extremely difficult.